The present invention relates generally to digital image processing, and particularly to a system and method for removing noise from color images using wavelets.
Digital color images are becoming a common part of every day life. This is due, in part, to the growing presence of desktop scanners, digital cameras, and digital video appliances in the consumer market. Desktop scanners, digital cameras, and digital video appliances are examples of tools that have greatly simplified the capture of digital color image information.
Tools that capture digital color image information predominantly use an electronic image sensor that senses an image. Typically the electronic image sensor is a solid-state device such as a charge-coupled device (CCD) or a complementary metal-oxide-semiconductor (CMOS) device. The image sensor connects to electronic interface circuitry which connects to a storage device and, optionally, to a display.
A typical electronic image sensor has many cells, also referred to as pixels, arranged along vertical and horizontal dimensions in a regular grid. These cells are sensitive to light intensities and convert incident light intensities to electrical signals which are subsequently represented in digital form. Electronic image sensors are made in many sizes such as, e.g. 400xc3x97300, 640xc3x97480, 1024xc3x97768, and 4096xc3x974096 pixels.
The image information sensed by each cell is also called a pixel. For example, a 640xc3x97480 CCD has about 307,200 pixels. After being converted to digital form, the image information (image data) is stored in a memory, typically an image memory. Image sensors having larger number of cells produce higher quality images; however, more pixel information must be processed and stored.
Typically, a digital signal processor processes the digital image data to improve the quality of the image. Various algorithms well-known in the art are used to improve image quality. Because there is such a large amount of image data, the image data is often compressed before storage in a storage medium.
Color imaging increases the complexity of processing the digital image data. In one method, the image sensor has a geometric arrangement of cells to respond to three colors, e.g, red, green and blue. Since each cell senses a particular color, various algorithms are used to interpolate the missing color information. Alternatively, two or more image sensors having different color sensitivity can be used and the image information is combined.
Digital images are subject to degradations caused by thermal noise from the image sensor. In one method, a wavelet transform is used to remove noise from gray-scale images. However, color images have properties not found in gray-scale images. Therefore, a system and method that uses the properties of color images for color image enhancement is desirable. In particular, this method should reduce the amount of noise in a color image.
A color image is enhanced by removing noise. A wavelet transform is applied to a color image to generate sets of wavelet transform coefficients for multiple channels. Edge maps are produced for the channels from the sets of wavelet transform coefficients. An edge likelihood map is generated based on at least one edge map. The edge likelihood map is applied to the sets of wavelet transform coefficients to generate sets of filtered wavelet transform coefficients.